It has hitherto been a fundamental fact that broadcast content and content on packaged media are used in a place where a receiving device and a reproduction device are disposed, or used in a device that is connected to these devices through a home network (hereinafter also referred to as “local access (LA)”. For example, it has been difficult from a technological point of view due to a communication path, a codec, and the like that, with a portable device, connection is made to a receiving device and a reproduction device from outdoors, and content is used after undergoing transmission via an external network, such as a WAN (Wide Area Network) (hereinafter also referred to as “remote access (RA)”. However, in the future, wide use of data communication technologies, such as LTE (Long Term Evolution) and WiMAX (World Interoperability for Microwave Access), and a high-compression codec of H.264 or the like is expected, and remote access can be implemented by utilizing these technologies. An example thereof is a usage in which a user accesses a server at a user's home from a place where the user is staying and reproduces content.
Additionally, digitized content is comparatively easily subjected to an illegal operation, such as copying and tampering. In particular, in remote access, a scheme for preventing illegal use involved in content transmission, that is, copyright protection, while permitting personal or home use of content, is necessary.
Examples of industry standard technologies for transmission protection of digital content include DTCP (Digital Transmission Content Protection) developed by the DTLA (Digital Transmission Licensing Administrator). In the DTCP, an authentication protocol to be used among devices at the time of content transmission, and a transmission protocol for encrypted content have been agreed upon. The regulations, in summary, are that a DTCP-compliant device does not send compressed content that is easy to handle outside of the device in a non-encrypted state, performs a key exchange that is necessary to decode encrypted content in accordance with a predetermined mutual authentication and key exchange (AKE) algorithm, and limit the range of a device that performs a key exchange in accordance with an AKE command. A server (source), which is a content providing source, and a client (sink), which is a content providing destination, share a key undergoing an authentication procedure by transmitting and receiving an AKE command, encrypt a transmission path by using the key, and perform the transmission of content. Therefore, since an unauthorized client cannot obtain an encryption key unless authentication with a server succeeds, it is not possible for the unauthorized client to enjoy content.
DTCP is primarily such that content transmission in a home network using a transmission path, such as IEEE 1394, has been defined. In recent years, as typified by the DLNA (Digital Living Network Alliance), also in a household, a trend of causing digitized AV content to be distributed through an IP network is in full swing. Accordingly, also in households, with the intent of causing digital content to be distributed through an IP network, development of a DTCP technology corresponding to an IP network, that is, DTCP-IP (DTCP mapping to IP), has been in progress.
DTCP-IP is a similar technology in that the DTCP technology is ported to an IP network, in which an IP network is used for a transmission path, and a protocol for content transmission, which is implemented in an IP network, such as an HTTP (Hyper Text Transfer Protocol) and an RTP (Real-Time Transfer Protocol), is used to transmit encrypted content. For example, in a case where content is to be transmitted in accordance with the procedure of HTTP, a source becomes an HTTP server, a sink becomes an HTTP client, a TCP/IP connection for HTTP is generated, and download transmission of encrypted content is performed (however, when upload transmission is to be performed, a source becomes an HTTP client, and a sink becomes an HTTP server).
The present DTCP-IP (DTCP Volume 1 Specification Supplement E Revision 1.2) is mainly intended to ensure use of content only in a household. For this reason, a round trip delay time (RTT) has been limited to a maximum of seven milliseconds for an AKE command, and the upper limit of the hop count (TTL: Time To Live) of an IP router has been set to three.
For example, an information communication system described below has been proposed: from when the source starts DTCP-IP authentication until immediately before it is completed, the source monitor continues to monitor each of received AKE commands, continues to update the maximum value of the TTL value, checks the maximum value of the TTL value immediately before the authentication procedure is completed, performs key exchange if the maximum value is smaller than or equal to three, and ends the authentication procedure when the maximum value exceeds three without performing processing of the final stage (refer to, for example, PTL 1).
However, if restrictions impose on RTT and TTL, it is not possible to access content whose copyright is protected, which exists in a server of a home network in a household, from a distant place outside the household.
When the convenience of a user is considered, it is desired to permit remote access to content, but it is contradictory to the profit of the content owner desiring copyright protection.